A disc drive is a digital data storage device which enables a user of a computer system to store and retrieve large amounts of data in a fast and efficient manner. Disc drives of the present generation have data storage capacities in excess of several gigabytes (GB) and can transfer data at sustained rates of several megabytes (MB) per second.
A typical disc drive is provided with a plurality of magnetic recording discs which are mounted to a rotatable hub of a spindle motor for rotation at a constant, high speed. An array of read/write heads is disposed adjacent surfaces of the discs to transfer data between the discs and a host computer. The heads are radially positioned over the discs by a rotary actuator assembly and a closed loop, digital servo system, and are caused to fly proximate the surfaces of the discs upon air bearings established by air currents set up by the high speed rotation of the discs.
A plurality of nominally concentric tracks are defined on each disc surface, with disc drives of the present generation having track densities in excess of 4,000 tracks per centimeter (10,000 tracks per inch). A preamp and driver circuit generates write currents that are used by the head to selectively magnetize the tracks during a data write operation and amplify read signals detected by the head during a data read operation. A read/write channel and interface circuit are operably connected to the preamp and driver circuit to transfer the data between the discs and the host computer.
The servo system operates in two primary modes: seeking and track following. During a seek, a selected head is moved from an initial track to a destination track on the corresponding disc surface using a velocity-control approach. The servo system initially determines a velocity profile indicative of the velocity trajectory the head should take based upon the length of the seek (in terms of tracks to go to the destination track). The servo system next applies current to an actuator coil to first accelerate and then decelerate the head toward the destination track in accordance with the velocity profile.
During the seek, the servo system sequentially measures (estimates) the actual velocity of the head and adjusts the current in relation to velocity error (i.e., the difference between the actual velocity and the target velocity as set forth by the velocity profile). As the head approaches the destination track, the servo system initiates a settle mode to bring the head to rest over the destination track within a selected settle threshold as a percentage of the track width, such as .+-.10% of track center. Thereafter, the servo system enters the track following mode wherein the head is nominally maintained over the center of the destination track until the next seek is performed.
As will be recognized, a disc drive is primarily utilized to transfer data between the tracks of the discs and the host computer. Such data transfer operations usually cannot occur during a seek, but rather require the drive to be in track following mode. Hence, to maximize disc drive data transfer rate capabilities, high performance disc drives are designed to achieve minimum average seek times. Typical drives of the present generation have nominal seek times on the order of eight milliseconds (msec), facilitating minimum sustained data transfer rates on the order of 20 MB/sec. It will be recognized that achieving a high as practicable minimum sustained data transfer rate is particularly important in sustaining data transfer operations, such as audio-video (AV) and communications applications.
In an effort to increase data transfer performance, designers of disc drives have in the past proposed various approaches which have met with greater or lesser amounts of commercial success. One particularly useful advancement in the art is zone based recording (ZBR) such as exemplified by U.S. Pat. No. 4,799,112 issued Jan. 17, 1989 to Bremmer et al., assigned to the assignee of the present invention. As will be recognized by those skilled in the art, ZBR generally entails defining a plurality of essentially constant bit-density zones across the radii of the discs, so that all of the tracks in each zone have the same number of data blocks (sectors) in which user data are stored. Thus, the number of data blocks per track increases in a step-wise fashion from the inner diameter (ID) to the outer diameter (OD) of the discs. The use of ZBR results in a disc drive transfer rate that varies with radius, with the transfer rate about doubling for tracks at the OD as compared to tracks at the ID.
Another advancement in the art is the use of multiple heads to access the same data surfaces on the discs. As exemplified by U.S. Pat. No. 5,218,496 issued Jun. 8, 1993 to Kaczeus, a head actuator structure is provided having pairs of heads which are angularly offset around each of the corresponding disc surfaces. U.S. Pat. No. 5,343,345 issued Aug. 30, 1994 to Gilovich also proposes a pair of read heads per disc surface, including both single actuator and dual actuator arrangements, with the dual actuators arranged on opposite sides of the disc stack.
Yet another advancement in the art is the use of a plurality of drives in a multi-drive array, sometimes referred to as a RAID ("Redundant Array of Inexpensive Discs"; also "Redundant Array of Independent Discs"). The array operates substantially as a single, large disc drive device. The primary impetus behind the development of such multi-drive arrays is the disparity between central processing unit (CPU) speeds which continue to increase at a phenomenal rate bounded primarily by electronic constraints, and disc drive input/output (I/O) speeds which are bounded largely by mechanical constraints. As will be recognized, an array of smaller, inexpensive drives functioning as a single storage device has been usually found to provide improved operational performance over a single, expensive drive. A seminal article proposing various RAID architectures was published in 1987 by Patterson et al., entitled "A Case for Redundant Arrays of Inexpensive Discs (RAID)", Report No. UCB/CSD 87/391, Dec. 1987, Computer Science Division (EECS), University of California, Berkeley, Calif.
RAID architectures are presently identified by numerical levels of organization, with each level providing different data integrity and I/O throughput characteristics. The particular level (or levels) used in a given application largely depend upon the requirements of the application, with commonly utilized RAID levels ranging from RAID 0 to RAID 7. However, a generally common characteristic of all RAIDs is that data transfer characteristics are enhanced over that of a single disc drive through striping data (i.e., storing portions of the data) across multiple drives in the array.
While these and other well known advancements in the art have allowed system designers to facilitate ever greater levels of disc drive data transfer performance, there remains a continual need for further improvements which enhance the minimum data transfer rate achievable by a disc drive. It is to this and other related ends that the present invention is directed.